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| United States Patent |
5,148,345
|
|
Allina
|
*
September 15, 1992
|
Prepackaged electrical transient surge protection
Abstract
Electrical transient surge protection of electrical equipment downstream
from a power source plus safeguarding of apparatus for providing such
surge protection against failure at high currents or high temperature.
Distributed-resistance fuse links facilitate operation of non-linear
over-voltage means, such as varistors, in clipping transient voltage
surges and shunting resulting currents to ground so as to protect
watt-hour meters and downstream loads from electrical surges, and to
safeguard such varistors from failure. Such apparatus may be located in
such diverse places as a power line weatherhead, a watt-hour meter base,
an adapter between a meter base and its normal mounting socket, or a
utility panel having such a socket, or in a circuit-breaker panel
downstream of the meter and upstream of metered loads.
| Inventors:
|
Allina; Edward F. (605 Capri Blvd., Treasure Island, FL 33706)
|
| [*] Notice: |
The portion of the term of this patent subsequent to March 6, 2007
has been disclaimed. |
| Appl. No.:
|
488677 |
| Filed:
|
March 5, 1990 |
| Current U.S. Class: |
361/104; 361/124; 361/127; D13/160 |
| Intern'l Class: |
H02H 009/04 |
| Field of Search: |
361/117,127,366,104,126,56,124
|
References Cited
U.S. Patent Documents
| 4439807 | Mar., 1984 | Reitz | 361/56.
|
| 4740859 | Apr., 1988 | Little | 361/56.
|
Primary Examiner: DeBoer; Todd E.
Attorney, Agent or Firm: McClure; Charles A.
Parent Case Text
This patent application is a continuation-in-part of my copending
application Ser. No. 286,446 filed Dec. 20, 1988, scheduled to issue as
U.S. Pat. No. 4,907,119, which itself was a continuation-in-part of each
of my following prior copending patent applications: Ser. No. 923,524
filed Oct. 28, 1986, to issue as U.S. Pat. No. 4,931,895; and Ser. No.
123,419 filed Jan. 12, 1988, now U.S. Pat. No. 4,901,187; and Ser. Nos.
185,584, now abandoned, and 185,587, the latter to issue as U.S. Pat. No.
4,866,560--both filed Apr. 22, 1988. The contents of those applications
are hereby incorporated herein by this reference.
Claims
I claim:
1. In a method of preventing transient electrical surges, received in a
circuit from an upstream power source, from going downstream, including
shunting resulting surge currents to ground via over-voltage means
including a disklike varistor with a conductive face,
the improvement comprising
providing a housing substantially surrounding such over-voltage means,
having at least one sidewall and furnished with leads from the power
source, and juxtaposing such varistor face adjacent the sidewall; and
providing, between the power source and the over-voltage means, conductive
fusible means including a distributed-resistance fuse link having its
electrical resistance distributed substantially evenly along its length,
establishing electrical contact of a side of such fuse link with such
varistor face, and sandwiching an end portion of such
distributed-resistance fuse link between such face of the varistor and
such sidewall.
2. Surge-protection method according to claim 1, including the step of
providing such end portion of such distributed-resistance fuse link
between such face of the varistor and such sidewall with a rectangular
transverse cross-section.
3. Apparatus adapted to protect electrical apparatus downstream from an
upstream power source subject to lightning or other electrical transient
source from resulting voltage surges in electrical power leads to such
downstream apparatus, comprising a housing having an electrically
non-conductive inside wall and, within the housing.
a varistor having a conductive face adjacent such wall, connectable between
such an electrical power lead and ground, and adapted to clip surge
voltages and to shunt resulting surge currents to an available ground, and
a fuse link having its electrical resistance distributed substantially
evenly along its length, connectable between such an electrical power lead
and such varistor, and in part contiguous with such varistor face and
sandwiched between the varistor and such adjacent wall.
4. Surge-protective apparatus according to claim 3, wherein
such fuse link is substantially rectangular in transverse cross section,
with one of its wide sides contiguous with such varistor.
Description
FIELD OF THE INVENTION
This invention relates to packaged electrical transient surge protectors
and especially physical modification thereof to improve their capability
of protecting downstream electrical equipment from being damaged by surge
currents in the tens of thousands of amperes.
BACKGROUND OF THE INVENTION
The present inventor has spearheaded the adoption of electrical transient
surge protectors in plug-in adapters inserted between the usual watt-hour
meter and its socket, as disclosed in his issued patents and other
copending patent applications identified above.
Similar protection against transient electrical surges may be installed
instead (or in addition) at nearby upstream locations, such as a
weatherhead, or downstream locations, such as a utility power panel, and
may either be built-in or be added thereto in prepackaged form.
Applicant's surge-protective apparatus utilizes varistors or equivalent
non-linear resistance means as components in their usual disklike form,
whether partially prepackaged or not.
A known type of downstream surge-protective device is often packaged in
so-called "grenade" form. An example of such device is disclosed by Reitz
as a "Secondary Arrester" in U.S. Pat. No. 4,439,807.
A common feature of packaged forms of surge protectors is lack of capacity
to carry extreme current densities, such as often result on power lines,
as from lightning. A metal oxide varistor can shunt considerable surge
current to ground and thereby protect downstream equipment, but repeated
surge conduction increases the likelihood of failure in associated
equipment or in surge-protection apparatus itself, if cooling time and
paths are inadequate. Varistors may get so hot therefrom as to reach a
characteristic failure temperature, resulting in loss of physical
integrity--perhaps explosively.
Rather than to rely upon the installers of prepackaged surge-protective
equipment to provide adequate local fusing to protect it from possible
overloading and failure, it is preferable to include in the package means
to increase its current-carrying capacity, to inhibit its temperature
rise, and lastly to disconnect it entirely.
The present inventor has pioneered increases in surge capacity and safety
of such surge-protective apparatus by heat-sinking component varistors (as
in the earliest filed of his above mentioned patent applications, to issue
as U.S. Pat. No. 4,931,895); by inserting temperature-responsive or
"thermal" fuses or similar cutoff devices to sense the temperature of the
varistors and to disconnect them from the power lines in the rare but
possible event of excessive temperature rise (as in his U.S. Pat. No.
4,866,560); by stacking varistor disks in parallel circuit therein (as in
his U.S. Pat. No. 4,901,187); and by including distributed-resistance fuse
links--with and without thermal cutoff means and/or varistor
stacking--between power line leads and such varistors (as in his U.S. Pat.
No. 4,907,119).
The present invention relates to extension of such improvements exemplified
in a prepackaged (such as a "grenade" ) type of surge-protective apparatus
but also suitable for use in a surge-protection device in a weatherhead,
meter adapter, utility panel, or elsewhere.
SUMMARY OF THE INVENTION
In general, the objects of the present invention are attained, in methods
of electrical transient surge protection utilizing over-voltage sensitive
means, such as varistors, by providing fuse links located between power
line leads and such varistors and having their electrical resistance
distributed substantially evenly along their entire length. Such usage is
compatible with various combinations of over-voltage, over-current, and
over-temperature protection, such as may include parallel stacking of
varistors, ground plane heat-sinking of varistors, and/or thermal fuse
safeguarding of varistors.
Apparatus of this invention preferably includes an electrically
non-conductive housing containing for each phase of an electrical power
source at least one varistor in disklike form connected at one face to a
power line phase lead through a distributed-resistance fuse link and
connected at the opposite face to an external ground. Exemplified here is
a conventional "grenade" housing of triangular or hexagonal cross-section
with alternating wide flat sides and relatively narrow arcuate sides (or
apex edges) capped at one end, and having protruding from its other end
electrical leads to external power and ground leads or terminals.
A primary object of the present invention is to provide heavy duty
electrical transient surge protection in a compact housing.
Another object of the invention is to enhance the capacity of varistors for
treating large transient electrical surges.
A further object is to provide improved over-current fusing of
surge-protective varistors shunting surge currents to ground.
Yet another object is to accomplish two or more of the foregoing objects
simultaneously and economically.
Other objects of the present invention, together with means and methods for
attaining the various objects, will become apparent from the following
descriptive text and the accompanying diagrams, all presented here by way
of example rather than limitation.
SUMMARY OF THE DRAWINGS
Figs. 1A to 1C represent schematically electrical transient
surge-protection embodiments of the prior art.
FIG. 1A is an electrical schematic diagram of primitive fusing of
electrical leads to a downstream location from a power source;
FIG. 1B is a similar schematic diagram including spark gaps as an
over-voltage means; and
FIG. 1C is a similar schematic diagram in which the spark gaps of FIG. 1B
have been replaced by variable-resistance over-voltage means, viz.,
varistors; and
FIG. 1D is a similar schematic diagram in which the varistors of FIG.. 1C
have been supplemented by distributed-resistance fuse links from the power
lines according to the present invention.
FIG. 2A is an exploded perspective view of an embodiment of this invention:
FIG. 2B is a plan view of the FIG. 2A apparatus (less cover);
FIGS. 3A, 3B, and 3C are enlarged transverse sectional detail views of
embodiments of distributed-resistance fuse link useful in apparatus of the
preceding views.
DETAILED DESCRIPTION
FIG. 1A shows schematically an arrangement 5 of the prior art. At the left
are power lines A, N, and B connecting (arrows at left) respectively to
leads 11, 15, and 19 directed (arrows at right) to electrical load
equipment downstream--not shown. Neutral line N is grounded; formerly it
was often missing and/or combined with one of the other lines. Leads 11
and 19 from respective phase lines A and B contain localized-resistance
fuses 6 (one each) of the prior art. Such an over-current device obstructs
electrical current flow much as a constriction in a fluid conduit
"bottle-necks" flow of fluid therethrough, absorbing energy from the flow
in doing so.
It will be understood that an additional phase line (not shown) is to be
added for three-phase power operation. An added phase line would have with
the same described features as phase line A or B.
A conventional fuse melts and opens the circuit when current (averaged over
a not-too-short and yet not-too-long period of time) occasions such an
I.sup.2 R energy transfer as to heat the fuse above its actuation
(melting) temperature. Inasmuch as electrical transients are exceedingly
brief even though extremely high in voltage, such a conventionally fused
arrangement limits surge-handling capability by fusing prematurely because
of its high-resistance "hot spots."
FIG. 1B shows similarly another arrangement 7 of the prior art, with spark
gaps 4 to grounded lead 15 from each of phase leads 11 and 19. Such
over-voltage means does not conduct at ordinary power voltages but only at
considerably higher voltage, whereupon it does provide some surge
protection. However, as both the breakdown and conduction voltages tend to
vary with the atmospheric conditions as well as the condition of the spark
gap, and as the arc temperature and follow-through current are
undesirable, especially at a power customer's location, such a primitive
arrangement is of marginal benefit and has become obsolescent except in
giant form on power line poles, towers, etc.
FIG. 1C shows more up-to-date arrangement 8 of the prior art, in which the
spark gaps have been replaced by varistors 10, commonly sintered metal
oxides or equivalent materials in disklike or wafer form, which conduct
very little at ordinary power voltages but conduct disproportionately well
at higher voltages. Such an arrangement provides substantial protection
for downstream equipment though not necessarily for the varistors, which
may increase in temperature from frequent surges that average out less
than the current tolerated by the conventional localized-resistance fuses.
Varistors have characteristic failure temperatures at which their
structural integrity is impaired, and above which they may suffer
catastrophic destruction--which itself may damage equipment nearby and
also may result in a fire that could be even more broadly damaging.
FIG. 1D shows arrangement 9 according to the present invention differing
from that of FIG. 1C of the prior art by addition of fuse links 14, one
each between power line leads 11 and 19 and varistors 10 from the
respective leads to ground lead 15. Unlike localized-resistance fuses 6 in
the respective phase leads, fuse links 14 have their resistance
distributed substantially evenly therealong. The fuse links are shown
schematically so as to suggest both greater diameter and length than
conventional fuses 6. A distributed-resistance fuse link is of
substantially lower resistance per unit length at surge pulse-widths than
such a localized-resistance fuse and is more likely to be jacketed rather
than enclosed in a housing.
A distributed-resistance fuse link is flexible and preferably has a
flexible insulating jacket of fabric or plastic film, so it can be bent to
fit between terminals spaced closer together than its straight end-to-end
length. Such a fuse link benefits surge protection equipment by the
manifest facility with which it conducts surge currents, by its tolerance
of extremely high currents of very short duration in transient surges, and
also by operating characteristics not hitherto recognized, such as
waveguide-like (e.g., skin effect) transmittal of very high frequencies
characteristic of transient surges, and also thermal sharing or
heat-sinking action because its large mass is more nearly comparable with
that of the varistors.
FIGS. 2A and 2B show embodiment 20 of electrical transient surge protection
means of this invention. FIG. 2A is in perspective and partially exploded
for clarity of assembly, whereas FIG. 2B is in plan, looking in on housing
22 and its contents before being closed off by cover 25 (indicated by
broken lines). For convenience of description, the end of housing 22
closed by the mentioned cover is taken as the top notwithstanding that
such end is shown lower than the opposite end (or bottom) in FIG. 2A. The
housing has open-ended externally threaded fitting 28 at its far (bottom)
end, which electrical leads 11, 15, and 19 pass through to an external
power source (not visible here).
Housing 22 is polygonal in transverse cross-section and is more
particularly substantially hexagonal, with three relatively wide flat
sidewalls 23 (one or more with a translucent window 13 therein)
alternating with a like number of relatively narrower arcuately curved
sidewalls (or apex edges) 27. Each flat sidewall is provided inside with
two widely spaced pairs of closely spaced narrow bosses 32. The bosses
nearer the center of the flat sidewall are shorter lengthwise than the
flanking ones. Varistor 10, shown exploded outside the housing in FIG. 2A,
is indicated in broken lines inside the housing and adjacent a sidewall in
FIG. 2B.
Visible in FIGS. 2A and 2B within the internal end of the base or bottom
fitting 28 are potting material 24 (electrically non-conductive but
thermally conductive), short ground terminal 35, emerging from the potting
material into and protruding through slot 38 in triangular disklike piece
26 of foamed insulation (when assembled), and an emergent pair of
distributed resistance fuse links 21 and 29. Disklike varistors 10 (each
with an insulating collar 10') rest edgewise upon the bosses adjacent
respective sidewalls inside the housing. The varistor face nearest the
wall overlies an end portion of an adjacent fuse link, in conductive
contact (electrical and thermal) with a flat side of such fuse link.
During assembly, three-lobed resilient grounding insert 33 is compressed
sufficiently to slide into the housing, where two of the lobes will
contact the available face of the respective varistors opposite the face
contiguous with the adjacent fuse link. Then the cover is fitted onto the
top of the housing and is sealed thereto in any suitable manner, as
adhesively, by dielectric heating, etc.
FIGS. 3A, 3B, and 3C show in transverse cross-section, and greatly
enlarged, alternative embodiments of distributed-resistance links 30A,
30B, and 30C according to this invention. Each fuse link has a thin
flexible covering 34 of insulation, composed of fabric, plastic film,
rubber, or the like. For good electrical contact with the varistors the
fuse links preferably have a rectangular section. Fuse link 30A is shown
as solid metal; fuse link 30B has hollow or slotted core 36B; and fuse
link 30C has a series of spaced tubular openings 36C extending
longitudinally therein.
A current overload sufficient to melt the fusible composition of the
distributed resistance fuse links (or other fusible material inside the
housing) also burns associated insulation and coats the translucent
housing window with soot, as can be confirmed at a glance, indicating that
the surge protection is no longer functioning and that the unit should be
removed for maintenance and/or be replaced by a fresh one. This seems
preferable to making the entire housing translucent, which would
necessitate cleaning or replacing the whole sooted housing.
The components and materials used according to this invention are
conventional in themselves and are readily available. Adequate housings
may be made of any of many polymeric materials of suitably high
degradation temperature and electrical non-conductivity. Epoxy resin or
other suitable polymer may be reinforced with glass fibers, if desired.
The illustrated housing is similar to that of Reitz U.S. Pat. No.
4,439,807 mentioned above and sold by General Electric Co.
Translucent--optionally transparent--windows for such housings may be made
of methyl methacrylate or similar materials meeting the requirements for
such non-conductive housings. Like the housing sidewalls, the windows are
conveniently several millimeters thick. Their combination of thickness and
clarity should be such that smoke deposited upon their inside surface will
be apparent from outside.
Varistors are available from well known sources, such as under the brand
name Panasonic. Other varistor sources include General Electric,
McGraw-Edison, and Siemens. Varistors are conveniently disklike and in
this described apparatus (and/or use) preferably are not covered, measure
about several centimeters in diameter, about several millimeters thick,
and weigh on the order of a dozen grams (plus or minus about half as much
for each measure or dimension).
Material for fuse links of distributed-resistance type is available in bulk
(sometimes called "fuse wire") from Action Fuse, Bussman, and Littlefuse,
for example. Such material in composition is rather similar to solder,
being composed of fusible material, such as an alloy mainly made up of tin
and lead, with perhaps traces of less common metals.
Where, as shown, such distributed-resistance fuse links are desired to be
contiguous with the varistor faces, rectangular transverse cross-sections
are favored over round--which may be preferred where other contact methods
and means are employed. In physical dimensions, such fuse links may range
from about a millimeter to several centimeters in thickness and width if
rectangular, and usually about several millimeters in diameter if
circular. The aspect ratio of such rectangular fuse links may range from
about 1:1 to as much as about 10:1 or so. An intermediate aspect ratio of
about 5:1 or 6:1 is generally suitable. The optimal cross-section depends
upon the type of circuitry used and/or the actual type of surge duty and
may be readily determined empirically.
The preferred length of such fuse links is a function of cross-section and
specific resistivity (resistance per unit length), as well as the desired
steady-state or "surgeless" current to be conducted. Although transient
surges are drastically different in amplitude and duration from
conventional power line frequencies and voltages, as a rule of thumb they
may be selected to correspond to a conventional rating, such as 30
amperes, at normal power conditions. A length from about several
centimeters to a dozen or so centimeters is suitable, and about a half
dozen centimeters is often suitable.
The regularity in resistance provided by such fuse links should not be
sacrificed--as otherwise may occur--when they are attached to terminals or
lead-in wires. Conventional crimping may impart an undesirable
high-resistance bottleneck to current flow--that under surge conditions
can fail, incapacitating an otherwise functional apparatus. "Nicropress"
or similar techniques previously used for non-electrical connections
should be employed to minimize distortion in connecting the relatively
soft distributed-resistance fuse links to other conductors, so as not to
introduce hot spots characteristic of localized-resistance over-current
fuses.
Variants in the apparatus and method of this invention have been suggested
in this specification, and other modifications may be made, as by adding,
combining, subdividing, or deleting parts or steps, while retaining at
least some of the advantages and benefits of the invention--which itself
is defined in the following claims.
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